Effect of challenge of pigs previously immunised with inactivated vaccines containing homologous and heterologous Mycoplasma hyopneumoniae strains.

BACKGROUND: Mycoplasma hyopneumoniae is the primary cause of enzootic pneumonia in pigs. Although vaccination is an important control tool, the results observed under field conditions are variable. This may be due to antigenic differences between the strains circulating in pig herds and the vaccine strain. This study compared the protective efficacy of four bacterins against challenge infection with a highly virulent field strain of M. hyopneumoniae. Seventy eight, one-week old piglets were randomly assigned to five treatment groups (A, B, C, D, E), 14 piglets each, and a negative control group (F) consisting of 8 piglets. All pigs were injected at 1 (D7) and 4 weeks of age (D28), with 2 ml of either a placebo or a bacterin based on selected M. hyopneumoniae strains, namely A (F7.2C), B (F20.1L), C (B2V1W20 1A-F), D (J strain), E (placebo; positive control), F (placebo; negative control). At D56, all pigs except those of group F were challenged intratracheally with 7 ml culture medium containing 107 CCU/ml of M. hyopneumoniae strain F7.2C. All pigs were euthanized and necropsied at D84. The severity of coughing and pneumonia lesions were the main parameters. Immunofluorescence (IF) testing, nested PCR testing of bronchoalveolar lavage (BAL) fluid and serology for M. hyopneumoniae were also performed. RESULTS: The different bacterins only slightly improved clinical symptoms (average 0.38 in vaccinated groups vs. 0.45 in group E) and histopathological lung lesions (average 3.20 in vaccinated groups vs. 3.45 in group E), but did not improve macroscopic lung lesions (score 4.30 vs. 4.03 in group E). None of the vaccines was significantly and/or consistently better or worse than the other ones. All bacterins evoked a serological response in the vaccinated animals. All pigs, except those from group F, were positive with nPCR in BAL fluid at D84. CONCLUSION: The bacterins did not induce a clear overall protection against challenge infection, and there were no significant differences in protective efficacy between bacterins containing homologous and heterologous M. hyopneumoniae strains. Further research is necessary to better characterize the antigens involved in protection and to elucidate the protective immunity responses following M. hyopneumoniae vaccination and/or infection.

Comparison of transmission of Mycoplasma hyopneumoniae in vaccinated and non-vaccinated populations.

A transmission experiment was performed to quantify the effect of vaccination on the transmission of Mycoplasma hyopneumoniae (M. hyopneumoniae) in nursery piglets by means of an adjusted reproduction ratio (R(n)). Thirty piglets, vaccinated at 1 week of age, and 30 non-vaccinated piglets, free of M. hyopneumoniae, were housed in six separate pens. In each pen, three animals that were intratracheally inoculated with M. hyopneumoniae, were housed together with seven contact piglets during the conventional nursery period of 6 weeks. At the end of the study, the infectious status of the animals was determined based on results of nPCR performed on bronchoalveolar lavage fluid. The R(n)-value in the vaccinated group was 2.38 (1.07-7.53) while in the non-vaccinated group, an R(n)-value of 3.51 (1.51-9.34) was observed, both not significantly different from each other (p=0.77). Under the actual experimental conditions, transmission of M. hyopneumoniae in nursery piglets was only numerically lower in vaccinated groups. In addition, vaccination with a conventional vaccine could not prevent the establishment of M. hyopneumoniae organisms in the lung.